[BUG] Bug with SKBlendMode.SrcOver

[TomQv]

Description

When drawing on an empty canvas with a half transparent color, then the resulting color is not identically to the paint anymore.
Example:

• background cleared with rgba(0,0,0,0)
• paint with color rgba(0,0,255,128), blue with 50% alpha, SKBlendMode.SrcOver
• result is rgba(0,0,128,128), so the blue was changed from 255 to 128

Code

var size = 32;
var buffer = new byte[size * size * 4];
var pinnedbuffer = GCHandle.Alloc(buffer, GCHandleType.Pinned);
var pointer = pinnedbuffer.AddrOfPinnedObject();

var imageinfo = new SKImageInfo(size, size, SKColorType.Rgba8888 , SKAlphaType.Premul);
using (var surface = SKSurface.Create(imageinfo, pointer))  
{
    var blendmodes = Enum.GetValues(typeof(SKBlendMode));
    var color = new SKColor(0, 0, 255, 128);

    foreach (var blendmode in blendmodes)
    {
        surface.Canvas.Clear(new SKColor(0, 0, 0, 0));
        var paint = new SKPaint
        {
            Style = SKPaintStyle.Fill,
            Color = color,
            BlendMode = (SKBlendMode)blendmode
        };
        surface.Canvas.DrawRect(0, 0, size, size, paint);

        // check first pixel
        var r = buffer[0];
        var g = buffer[1];
        var b = buffer[2];
        var a = buffer[3];
        //Debug.WriteLine(string.Format("alpha={0} blue={1} mode={2}", a, b, blendmode.ToString()));
        if (a == color.Alpha  && b == color.Blue )
        {
            // ok
        }
    }            
}

Expected Behavior

Drawing with source rgba(0,0,255,128) with SrcOver on an emtpy canvas destination rgba(0,0,0,0) should result in unchanged color.

Actual Behavior

Its not, result is rgba(0,0,128,128) instead of rgba(0,0,255,128).

I actually tried all SKBlendmodes, but none of them is producing the expected behaviour.

Version of SkiaSharp

2.88.3 (Current)

Last Known Good Version of SkiaSharp

Other (Please indicate in the description)

IDE / Editor

Visual Studio (Windows)

Platform / Operating System

Windows

Platform / Operating System Version

Windows

Devices

No response

Relevant Screenshots

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Relevant Log Output

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Code of Conduct

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